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JSDoc tag

@const and @constant

  • Not checked by TypeScript
  • Tested with TypeScript 5.9
  • Alias @constant

@const (also @constant) is a JSDoc documentation tag for a value that should not change. TypeScript 5.9 does not read it: a let tagged @const can be reassigned without an error, and @constant {number} does not even apply its type.

The const keyword is what TypeScript enforces. For an object or array whose contents should also stay fixed, add a const assertion, /** @type {const} */ (value), which makes every property read-only and keeps the literal values as types.

At a glance

Use JSDoc in a .js file TypeScript in a .ts file
A variable that never changes const maxItems = 10; const maxItems = 10;
An object whose values never change const limits = /** @type {const} */ ({ items: 10 }); const limits = { items: 10 } as const;

Examples

limits.js TypeScript 5.9.3 TypeScript 5.9.3, run 21 October 2026: No errors. 6.0.3 TypeScript 6.0.3, run 21 October 2026: the same as 5.9.3. No errors. 7.0.2 TypeScript 7.0.2, run 21 October 2026: the same as 5.9.3. No errors. checked 21 October 2026 
/** @const */
export let maxItems = 10;

/** @constant {number} */
export let maxWeight = 25;

// TypeScript accepts both: neither tag is read.
maxItems = 50;
maxWeight = 30;
@const does not stop a reassignment.
jsconfig.json for this example 
{
  "compilerOptions": {
    "checkJs": true,
    "noEmit": true,
    "strict": true,
    "target": "es2022",
    "module": "nodenext",
    "types": []
  },
  "include": ["*.js"]
}
limits.js limits.ts Error confirmed 5.9.3 TypeScript 5.9.3, run 21 October 2026: limits.js(4,1): error TS2588: Cannot assign to 'maxItems' because it is a constant. limits.ts(4,1): error TS2588: Cannot assign to 'maxItems' because it is a constant. 6.0.3 TypeScript 6.0.3, run 21 October 2026: the same as 5.9.3. limits.js(4,1): error TS2588: Cannot assign to 'maxItems' because it is a constant. limits.ts(4,1): error TS2588: Cannot assign to 'maxItems' because it is a constant. 7.0.2 TypeScript 7.0.2, run 21 October 2026: the same as 5.9.3. limits.js(4,1): error TS2588: Cannot assign to 'maxItems' because it is a constant. limits.ts(4,1): error TS2588: Cannot assign to 'maxItems' because it is a constant. checked 21 October 2026 
export const maxItems = 10;

maxItems = 50;error TS2588: Cannot assign to 'maxItems' because it is a constant.
export const maxItems = 10;

maxItems = 50;error TS2588: Cannot assign to 'maxItems' because it is a constant.
The const keyword is what TypeScript enforces.
jsconfig.json for this example 
{
  "compilerOptions": {
    "checkJs": true,
    "noEmit": true,
    "strict": true,
    "target": "es2022",
    "module": "nodenext",
    "types": []
  },
  "include": ["*.js"]
}

How we tested it

The first example tags one let with @const and another with @constant {number}, then assigns to both. If TypeScript honored either tag, the assignments would be errors. tsc reports nothing. The second example uses the keyword, and the assignment fails with TS2588 ("Cannot assign to 'maxItems' because it is a constant").

The keyword and the const assertion

const stops reassignment of the variable, not changes to the object it points to: const limits = { items: 10 } still allows limits.items = 50. /** @type {const} */ ({ items: 10 }) makes items read-only and types it as the literal 10; TypeScript 4.5 added this JSDoc form of as const.

Neither is a run-time freeze. Object.freeze stops changes at run time too, and TypeScript types its result as Readonly.

Gotchas

  • Do not confuse the @const tag with @type {const}. The first is ignored; the second is a const assertion that TypeScript does read.
  • @constant {number} looks like it declares a type, but TypeScript does not take the type from it. Use @type {number} for that.

Questions

Is there a JSDoc equivalent of as const?

Yes: a cast to const, written /** @type {const} */ (expression), with the parentheses. It works since TypeScript 4.5.

  • JSDoc tag

    @type

    The JSDoc @type tag gives a variable, property or expression a type that TypeScript checks: the syntax, casts with parentheses, and whole function types.

  • JSDoc tag

    @readonly

    The JSDoc @readonly tag lets a class property be set only while the object is built. Where it works, why it breaks a @typedef, and what to write instead.

  • JSDoc tag

    @enum

    The JSDoc @enum tag types an object whose members share one type. How TypeScript reads it, why it is not a TypeScript enum, and a const object instead.

  • JSDoc tag

    @satisfies

    The JSDoc @satisfies tag (TypeScript 5.0 and later) checks a value against a type but keeps the value's own, narrower type. When to use it instead of @type.

  • Type expression

    const assertions

    /** @type {const} */ (value) is the JSDoc form of as const, since TypeScript 4.5: literal types kept, properties read-only, arrays turned into read-only tuples.

Sources

Last reviewed 20 October 2026.